1. Field of the Invention
The present invention relates to a method of polishing a substrate having an interconnect-forming structure including a metal film, a barrier film, and an insulating film, and more particularly to a polishing method in which polishing of the substrate is terminated when the insulating film is removed by a predetermined thickness.
2. Description of the Related Art
In an interconnect formation process of a semiconductor wafer, a metal film is formed to provide interconnects and then chemical mechanical polishing (CMP) is performed to remove the excess metal film that is not used for the interconnects. In this polishing process, after the metal film is removed, a barrier film, underlying the metal film, is polished. Further, a hardmask film, underlying the barrier film, is polished. The polishing operation is terminated when the hardmask film reaches a predetermined thickness.
The hardmask film is an insulating film composed of an insulating material. This hardmask film is formed so as to cover an interlevel dielectric. The interlevel dielectric has an insulating property as well as the hardmask film, and is typically composed of a low-k material which is a brittle material. The purpose of forming the hardmask film is to protect this interlevel dielectric from physical processing by CMP.
FIG. 1 is a cross-sectional view showing an example of a multilayer structure for forming interconnects. As shown in FIG. 1, a hardmask film 903 is formed on an interlevel dielectric 902 which is composed of SiO2 or low-k material. The hardmask film 903 is typically composed of SiO2. Via holes 904 and trenches 905 are formed in both the hardmask film 903 and the interlevel dielectric 902 by a lithography etching technique or the like. Further, a barrier film 906, composed of metal such as Ta or TaN, is formed on surfaces of the hardmask film 903, the via holes 904, and the trenches 905 by sputtering or the like. Further, copper plating is performed on a substrate to fill the via holes 904 and the trenches 905 with copper to form a copper film 907 as a metal film on the barrier film 906.
Thereafter, chemical mechanical polishing (CMP) is performed to remove the copper film 907 and the barrier film 906. Polishing is terminated when the hardmask film 903 reaches a predetermined thickness, as indicated by a dotted line in FIG. 1, whereby the interconnects, composed of copper in the via holes 904 and the trenches 905, are formed.
The purpose of polishing the barrier film, which is a metal film, is to prevent short circuit between the interconnects. This is the same as the purpose of removing the metal film (i.e., the copper film 907). On the other hand, the purpose of polishing the hardmask film is to remove damaged portions or degenerated portions thereof resulting from the etching process that has been performed when forming the via holes and the trenches.
Furthermore, an interconnect resistance of the metal interconnects can be controlled by polishing the hardmask film. Specifically, when the barrier film is removed, separation between the interconnects is completed, whereby the short circuit will no longer occur. Therefore, the interconnects are formed at this stage. Polishing is further continued even after the formation of the interconnects so as to reduce cross sections of the interconnects to thereby change the interconnect resistance. This is because, when the hardmask film is polished, the metal interconnects is also polished simultaneously.
Management of the interconnect resistance is an important factor for device fabrication. That is, management of an amount of the hardmask film polished, i.e., a height of the metal interconnects, is very important in view of securing stability of a device. Therefore, highly accurate polishing end point detection is required in polishing of the hardmask film and the metal interconnects. For example, it is necessary to detect a polishing end point to an accuracy of a target thickness ±5 nm to ±10 nm.
Since the insulating film, such as the hardmask film and the interlevel dielectric, has a light transmission property, an optical means has been conventionally used as a film-thickness measuring method for the insulating film. However, both of the hardmask film and the interlevel dielectric may be formed by SiO2 which can transmit light therethrough. In such a case, even if light is applied to the hardmask film during polishing thereof, it may be difficult to measure a film thickness due to an effect of the underlying interlevel dielectric. A failure in measuring of the film thickness results in a failure in terminating the polishing operation based on when the hardmask film reaches a predetermined thickness. Moreover, application of light to the metal interconnects, such as copper, can cause the metal of the interconnects to corrode. Such a phenomenon is called photo-corrosion.